Multipurpose air-cooled four-cycle engines have various requirements depending on what sort of working machine the engine is to serve. A compressor, an axial-flow pump, or an outboard engine directly connected to the propeller shaft requires high-speed revolution, so for these applications the crankshaft is used as the output shaft. Much farm equipment, on the other hand, requires low-speed output. Since the camshaft has a rotary speed half that of the crankshaft, it is used as the output shaft for this sort of application.
We shall now discuss, with reference to FIGS. 6 and 7, an example of the prior art, a four-cycle engine in which the crankshaft is used as the output shaft. (Reference: Japanese Patent Publication (Kokai) Hei 1-193433).
FIG. 6 is a frontal cross section of a four-cycle overhead-valve engine. FIG. 7 is a lateral cross section. In these drawings, 1 is the crankcase; 2 is the cylinder with its cooling fins; 2a is the lower end of cylinder 2; 3 is the piston which slides up and down against the interior wall of the aforesaid cylinder; 4 is the connecting rod which joins piston 3 and crank pin 5b; and 5 is the crankshaft. The crankshaft 5 comprises shaft portions 5c.sub.1 and 5c.sub.2, crank arms 5a and crank pin 5b. The said shaft portions 5c.sub.1 and 5c.sub.2 are supported at two points by bearings mounted on crankcase 1. Gear 6, which drives the cam, is attached to shaft portion 5c.sub.1 on the output side of the aforesaid crankshaft 5. The rotary force of crankshaft 5 is transmitted to camshaft 7 via cam gear 8. Exhaust/intake valves 11 are opened and closed by the operation of cams 7a, tappets 9 and rocker arms 10.
Camshaft 7 is also supported at two points by sliding bearings mounted on crankcase 1.
Governor G is engaged with cam drive gear 6 in crankcase 1. It detects the speed at which crankshaft 5 is rotating and controls the speed of rotation. Flywheel 12 is mounted on shaft portion 5c.sub.2, which protrudes from crankcase 1 to the exterior on the side of the crankshaft opposite the output side.
In engines which employ the camshaft as the output shaft, the standard arrangement used in industry in order to standardize multipurpose engines is to have the output shaft rotate to the left if viewed from the output side. In an engine in which crankshaft 5 and camshaft 7 are linked via gears 6 and 8 (hereafter referred to as the "gear transmission scheme"), as can be seen in the aforementioned FIGS. 6 and 7, crankshaft 5 must be made to rotate in the direction opposite that in which it would rotate if it were the output shaft. Accordingly, a flywheel, a recoil starter or the like must be employed to provide opposite rotation. This has the result of increasing the number of different types of components needed (i.e., the parts count). Furthermore, a number of parts must be added in order to use the part for both leftward and rightward rotation, such as shrouds to guide the cooling air. This must become the engine larger to accommodate the extra parts.
To address the shortcomings of the above-described gear transmission scheme in an engine which employs the camshaft as the output shaft, a chain transmission scheme has been suggested by which the camshaft and the crankshaft are connected by means of two sprockets and a chain. This chain transmission scheme has the benefit that the camshaft and crankshaft have the same direction of rotation. On the other hand, when the camshaft is used as the output shaft, the entire output is transmitted from the crankshaft to the camshaft through the chain, so when the chain stretches a timing lag may result. The shock which occurs when engine braking is applied also poses a problem, as does noise. Lining up the crankshaft and camshaft during assembly (i.e., matching the timing) is also problematic.
To address the problem of chain noise, the use of a silent chain has been suggested. (Japanese Utility Model Publications Showa 60-178645, Heisei 6-43396 and Showa 64-17054).
An example of a silent chain is pictured in FIG. 3.
FIG. 3(A) is a lateral view of the chain with the sprocket engaged (In (A) guide plates 274 are not shown.). (B) is a cross section taken along line A--A. (C) is a plan view. In the drawings, 25a (26a) is the crown of the gear tooth of sprocket 25 (26). 271 and 272 are link plates. As can be seen in (A), there is a depression 272a in the middle of each plate in which the aforesaid crown of the gear tooth 25a (26a) engages. The left and right sides of the plate project like tongues to correspond to the crown of the gear tooth. Pin holes 272b are formed in the upper portion of the plate.
As can be seen in FIGS. 3(B) and (C), the said link plates 271 and 272 are arranged in three layers. Plate 271, which forms the middle layer, is placed at a distance from plates 272 which is equal to one pitch 25a (26a) of the crown of the gear tooth. This entire structure is supported at guide plates 274, which are on either side of it, by pin 273 in such a way that it is free to rotate.
To be more specific, there are three link plates, 272, 271 and 272, which are standing in a row. The link plates 272 to the right and left are offset with respect to the central link plate 271 by a single pitch of the crowns of the gear teeth along the length of the chain. The lateral surfaces of the three link plates 271 and 272 partially overlap each other. In this way a large number of link plates is connected lengthwise to form a chain.
As is shown in FIGS. 4(b) and (c), guide plates 274 are shaped like segments of a ring. The said plates 274 are provided on both sides of the chain and, as can be seen in (b), they are placed on either side of the aforesaid gear teeth 25a (or 26a). The purpose of these guide plates 274 is to insure that the aforesaid link plates 271 and 272 cannot shift laterally with respect to gear teeth 25a (or 26a) of sprocket 25 (or 26).
The aforesaid link plates 271 and 272 and guide plates 274 are linked together by pins 273, which are inserted into holes 272b once the holes in the link plates have been aligned with those in the guide plates. These pins support the plates axially in such a way that they are free to rotate. This constitutes the configuration of silent chain 27.
When the silent chain is used in the chain transmission scheme, and the cam shaft is used as the output shaft, as has been described above, we can solve two problems inherent in other transmission schemes. One is the proliferation of components required by gear transmission schemes because of the reverse rotation of the crankshaft or, if single components are made to serve both rotations, the resulting bulkiness of the engine. This can be ameliorated and the engine can be made smaller. The other problem, which is inherent to chain transmission schemes, is noise. This too can be solved by the present scheme. However, a number of problems remain: for example, the timing can shift as the chain stretches; a shock is generated when engine braking is applied; and it is difficult to match the phases of the crankshaft and camshaft (i.e., to match their timing) when the engine is being assembled.